WIP: Introduce deadweight for tracking unused css selectors

[dmitrytrager]

This PR is created according to #225

Added Rack middleware is based on that described in this article. Although I don't see any reason to insert it exactly after Rack::Lock.

This PR inserts Deadweight as a last step of bin/quality script. It also precompiles assets in test env since we need to provide compiled css for deadweight.

Main drawback here is following:

• this solution looks for css used in response data
• response data is collected as a result of requests to application
• requests are triggered by tests
• as a result, current tests can provide not all the possible responses from the app and we can treat some selectors as 'unused' mistakenly

[timurvafin]

@dmitrytrager I believe you need involve @maratfakhreev or @rustemginiyatullin to review this PR.

[dmitrytrager]

@timurvafin that's for sure. I just think that time did not come yet)

[maratfakhreev]

@dmitrytrager looks awesome) Could you show how this awesome tool works? Tomorrow for example.

[timurvafin]

@dmitrytrager what's the status? Please reopen when it will be updated.